JP2011257689A - Diffraction optical element, optical system and optical equipment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a diffraction optical element, an optical system and optical equipment suppressing unnecessary light.SOLUTION: The diffraction optical element includes a thin film 20 disposed at least a part of a boundary face between a pair of diffraction gratings 11, 12, and is characterized in that a value obtained by subtracting the larger refractive index of the refractive indices of the pair of diffraction gratings from the refractive index of the thin film is greater than 0.5 and smaller than 0.8.

Description

  The present invention relates to a diffractive optical element, an optical system, and an optical apparatus that are used for a lens of an optical system.

  In a diffractive optical element used for a lens of an optical system, two diffraction gratings are arranged in close contact, and the material and the grating height that constitute each diffraction grating are set appropriately to obtain high diffraction efficiency in a wide wavelength band. Are known. When a light beam enters the diffractive optical element having the grating surface and the grating wall surface, the incident light beam is reflected or refracted by the grating wall surface, thereby generating unnecessary light (flare). Patent Documents 1 and 2 propose a diffractive optical element in which an absorption film is provided on a grating wall surface in order to suppress unnecessary light (flares) on the grating wall surface. Patent Documents 3 and 4 propose that two diffraction gratings are arranged in close contact, and a thin film is provided on the boundary surface for the purpose of improving adhesion. Patent Document 5 discloses calculation of diffraction efficiency using a rigorous coupled wave analysis (RCWA: Regulated Coupled Wave Analysis).

JP 2003-240931 A JP 2004-126394 A JP 2004-13081 A JP 2005-62717 A JP 2009-217139 A

  In a diffractive optical element used for a lens of an optical system, unnecessary light that is particularly problematic is a high refractive index medium and a low refractive index medium due to a light beam incident at an oblique incident angle (off-screen light incident angle) different from the designed incident light beam. Unwanted light caused by total reflection occurring at the interface. However, since Patent Documents 1 to 4 do not examine this, there is a possibility that the effect of suppressing unnecessary light is not sufficient.

  Accordingly, an object of the present invention is to provide a diffractive optical element, an optical system, and an optical apparatus that suppress unnecessary light.

  The diffractive optical element of the present invention is a diffractive optical element used for a lens of an optical system, and is formed of different materials and stacked in the optical axis direction, the first diffraction grating and the second diffraction grating, The first diffraction grating and the second diffraction grating are arranged in at least part of a boundary surface between the first diffraction grating and the second diffraction grating, and the first diffraction grating and the second diffraction grating are composed of a single layer or a multilayer made of different materials, A thin film that is transparent to light in the wavelength band used, the refractive index nd1 of the first diffraction grating material for the d-line, the refractive index nd2 of the second diffraction grating material for the d-line, and the thin film The maximum refractive index nd3 with respect to the d-line of the material constituting one layer satisfies the following formula.

nd1 <nd2
0.5 <nd3-nd2 <0.8

  According to the present invention, it is possible to provide a diffractive optical element, an optical system, and an optical apparatus that suppress unnecessary light.

It is the top view and side view of a diffractive optical element. Example 1 It is a partial expanded sectional view of FIG. Example 1 FIG. 2 is a partially enlarged perspective view of a diffraction grating portion shown in FIG. Example 1 FIG. 3 is a partially enlarged sectional view of FIG. 2. Example 1 FIG. 2 is an optical path diagram of an optical system having the diffractive optical element shown in FIG. 1. Example 1 It is a schematic diagram for demonstrating the influence of unnecessary light in the optical system shown in FIG. Example 1 6 is a graph of diffraction efficiency of the diffractive optical element shown in FIG. Example 1 It is a graph as a comparative example with respect to FIG. It is a schematic diagram for demonstrating the problem in the comparative example shown in FIG. It is a schematic diagram for demonstrating the effect of the diffractive optical element shown in FIG. Example 1 It is a graph of the diffraction efficiency with respect to the design incident light beam of a diffractive optical element. Example 1 It is a graph of the diffraction efficiency with respect to the design incident light beam of a diffractive optical element. (Comparative example) It is a graph of the diffraction efficiency with respect to the off-screen incident -10 degree light beam of a diffractive optical element. Example 1 It is a graph of the diffraction efficiency with respect to the off-screen incident -10 degree light beam of a diffractive optical element. (Comparative example) It is a graph of the diffraction efficiency with respect to the off-screen incident +10 degree light beam of a diffractive optical element. (Example 2) It is a graph of the diffraction efficiency with respect to the design incident light beam of a diffractive optical element. (Example 2) It is a graph of the diffraction efficiency with respect to the off-screen incident -10 degree light beam of a diffractive optical element. (Example 2) It is a graph of the diffraction efficiency with respect to the off-screen incident +10 degree light beam of a diffractive optical element. (Example 3) It is a graph of the diffraction efficiency with respect to the design incident light beam of a diffractive optical element. (Example 3) It is a graph of the diffraction efficiency with respect to the off-screen incident -10 degree light beam of a diffractive optical element. (Example 3) It is a partial expanded sectional view of a diffractive optical element. Example 4 It is a graph of the diffraction efficiency with respect to off-screen incident +10 degree light beam of the diffractive optical element shown in FIG. Example 4 It is a graph of the diffraction efficiency with respect to the design incident light beam of a diffractive optical element. Example 4 It is a graph of the diffraction efficiency with respect to the off-screen incident -10 degree light beam of a diffractive optical element. Example 4 It is a partial expanded sectional view of a diffractive optical element. (Example 5) FIG. 26 is a graph of diffraction efficiency of the diffractive optical element shown in FIG. (Example 5) It is a graph of the diffraction efficiency with respect to the design incident light beam of a diffractive optical element. (Example 5) It is a graph of the diffraction efficiency with respect to the off-screen incident -10 degree light beam of a diffractive optical element. (Example 5)

  Embodiments of the present invention will be described below with reference to the accompanying drawings.

  FIG. 1 is a plan view and a side view of a diffractive optical element (DOE) 1 according to the first embodiment. The DOE 1 is configured to increase the diffraction efficiency of diffracted light of one specific order (hereinafter also referred to as “specific order” or “design order”) in the use wavelength region of the entire visible wavelength body.

  The DOE 1 includes a pair of transparent substrates 2 and 3 and a diffraction grating portion 10 disposed therebetween. Each of the substrates 2 and 3 has a shape exhibiting a flat plate or lens action, but in this embodiment, the upper and lower surfaces of the substrate 2 and the upper and lower surfaces of the substrate 3 have curved surfaces, respectively.

  The diffraction grating portion 10 has a concentric diffraction grating shape centered on the optical axis O, and exhibits a lens action. 2 is a partially enlarged cross-sectional view of the vicinity of the central portion of FIG. 1, and FIG. 3 is a partially enlarged perspective view of the diffraction grating portion 10. FIG. 4 is an enlarged cross-sectional view of FIG.

  In order to make the lattice shape easy to understand, FIGS. 2 to 4 are considerably deformed in the lattice depth direction, and the number of lattices is also drawn smaller than the actual number. 3 and 4, an incident light beam a is a light beam incident at an incident angle of 0 degrees, which is the designed incident angle of the DOE 1. The incident light beam b is a light beam incident downward at an oblique incident angle (off-screen light incident angle). The incident light beam c is a light beam incident upward at an oblique incident angle (off-screen light incident angle).

  1 and 3, the diffraction grating portion 10 is formed by bringing a diffraction grating (first diffraction grating) 11 and a diffraction grating (second diffraction grating) 12 into close contact with each other in the optical axis direction. And 12 are provided with a transparent thin film 20 in the used wavelength band. The diffraction grating 11 may be integrated with the substrate 2 or may be a separate body. The diffraction grating 12 may be integrated with the substrate 3 or may be a separate body.

  In this embodiment, the diffraction gratings 11 and 12 are in close contact with each other in the optical axis direction. However, the intervening thin film 20 may be provided over the entire boundary surface between the two as described later. It is sufficient if the gratings 11 and 12 are laminated in the optical axis direction. In this embodiment, no space is provided between the diffraction gratings 11 and 12.

  The diffraction grating 11 has a concentric blazed structure composed of a grating surface 11a and a grating wall surface 11b, and the diffraction grating 12 has a concentric blazed structure composed of a grating surface 12a and a grating wall surface 12b. Each diffraction grating 11 and 12 realizes a lens action (light convergence action or diverging action) by gradually changing the grating pitch from the optical axis O toward the outer peripheral portion.

  The grating surfaces 11a and 12a and the grating wall surfaces 11b and 12b are in contact with each other without a gap, and the diffraction gratings 11 and 12 function as a single DOE 10 as a whole. Further, by using the blazed structure, incident light incident on the DOE 1 is concentrated in a specific diffraction order (+ 1st order in the figure) direction with respect to the 0th-order diffraction direction that is transmitted without being diffracted by the diffraction grating unit 10. Diffraction.

  Since the working wavelength region of the DOE 1 of the present embodiment is the visible region, the material and the grating height constituting the diffraction gratings 11 and 12 are selected so that the diffraction efficiency of the diffracted light of the designed order is high in the entire visible region. ing. That is, the maximum optical path length difference of light passing through a plurality of diffraction gratings (diffraction gratings 11 and 12) (the maximum value of the optical optical path length difference between the peaks and valleys of the diffractive portion) is in the vicinity of an integral multiple of the wavelength within the operating wavelength range. Thus, the material and grating height of each diffraction grating are determined. Thus, by appropriately setting the material and shape of the diffraction grating, high diffraction efficiency can be obtained over the entire wavelength range.

  In general, the grating height of the diffraction grating is defined by the height of the grating tip and the grating groove in a direction (plane normal direction) perpendicular to the grating period direction. Further, when the lattice wall surface is shifted from the surface normal direction or when the lattice tip is deformed, it is defined by the distance between the extended line of the lattice surface and the intersection of the surface normal. The diffraction grating material and the grating height are not limited.

The diffraction grating 11 is made of a fluorine-acrylic ultraviolet curable resin (nd = 1.504, νd = 16.3, θgF = 0.390, n550 = 1.511) mixed with ITO fine particles. The diffraction grating 12 is made of an acrylic ultraviolet curable resin (nd = 1.567, νd = 47.0, θgF = 0.570, n550 = 1.570) mixed with ZrO ₂ fine particles. Note that nd of each of the diffraction gratings 11 and 12 is a refractive index with respect to d-line, νd is an Abbe number with respect to d-line, θgF is a partial dispersion ratio with respect to g-line and F-line, and n550 is a refractive index with respect to a wavelength of 550 nm.

  In this embodiment, the diffraction gratings 11 and 12 are made of different materials, the diffraction grating 11 is made of a low refractive index dispersion material, and the diffraction grating 12 is made of a high refractive index dispersion material having a higher refractive index. Has been. However, it is sufficient that either the refractive index of the material of the diffraction grating 11 with respect to the d-line or the refractive index of the material of the diffraction grating 12 with respect to the d-line is larger.

  The resin material in which the fine particles are dispersed is an ultraviolet curable resin, and includes any organic resin of acrylic, fluorine, vinyl, and epoxy, but is not particularly limited. In this embodiment, the design order is + 1st order, but the same effect can be obtained even if the design order is other than + 1st order, and the design order is not limited to the design order.

  The fine particles include, but are not particularly limited to, oxides, metals, ceramics, composites, and mixtures. The average particle diameter of the fine particle material is preferably ¼ or less of the wavelength (use wavelength or design wavelength) of light incident on the diffractive optical element. When the particle diameter is larger than this, there is a possibility that Rayleigh scattering becomes large when the fine particle material is mixed with the resin material.

  Instead of the resin material in which the fine particles are dispersed, an organic material such as a resin material, a glass material, an optical crystal material, a ceramic material, or the like may be used.

  Moreover, you may control for every ring zone by changing the width | variety or shape of a thin film for every ring zone of a diffractive optical element. As a result, unnecessary light reaching the imaging plane can be effectively suppressed.

  The thin film 20 has a substantially uniform thickness along the grating wall surface, reduces unnecessary light that is generated by obliquely incident (off-screen incident) light flux and reaches the imaging surface, and light in a wavelength band used by the diffractive optical element. It is a transparent film. The thin film 20 is composed of a single layer or multiple layers. In this embodiment, the thin film 20 is composed of a single layer.

  The thin film 20 is disposed on at least a part of the boundary surface between the diffraction gratings 11 and 12, and is provided on the grating wall surfaces 1b and 1b 'in this embodiment. The grating height d shown in FIG. 4 of the grating wall surfaces 1b and 1b ′ is 9.29 μm, and the design order is + 1st order.

The thin film 20 is made of a material having a high refractive index different from that of the diffraction gratings 11 and 12, and is made of TiO ₂ (refractive index n = 2.323 for the d-line) in this embodiment.

  When the thin film 20 has a multilayer structure, it is preferable that the maximum refractive index with respect to the d-line of the material constituting the single layer 20 be 1.3 or more and 2.5 or less. However, since the refractive index of the thin film must be 0.5 or more larger than the refractive index of the diffraction gratings 11 and 12 as shown in the mathematical formulas described later, about 2.0 or more is required. The reason why the value is 2.5 or less is that it is difficult to select a material in consideration of the refractive index of the existing material.

  The thickness or width w of the thin film 20 in the direction perpendicular to the lattice wall surface, which is the lamination surface, is 0.1 μm.

  The manufacturing method of the thin film 20 is not specifically limited. For example, the diffraction grating 12 is manufactured, and then the thin film 20 is selectively formed. Specifically, it is possible to use a method in which a material constituting the thin film is formed into a thin film shape by a vacuum deposition method or the like, and then patterned by a lithography method or a nanoimprint method, and selectively formed by an etching method or the like. . Alternatively, a method of selectively forming by a vapor deposition method or the like using a mask pattern can be used. Thereafter, the diffractive optical element can be manufactured by forming the diffraction grating 11. Since the thin film 20 can be manufactured by a process such as vapor deposition, the thin film 20 can be manufactured at a lower cost and more easily than manufacturing the absorption film described in Patent Documents 1 and 2.

  FIG. 5 shows a telephoto imaging optical system using DOE 1 that can be applied to an imaging apparatus (camera, etc.), f = 392.00 mm, fno = 4.12, half angle of view 3.16 degrees, A diffractive surface is provided on the surface. FIG. 6 is a schematic diagram of unnecessary light in the optical system of FIG.

  In FIG. 5, reference numeral 30 denotes a photographing lens, which has an aperture 40 and a DOE 1 therein. The diaphragm 40 is disposed behind the DOE 1. Reference numeral 41 denotes a film which is an imaging surface or a photoelectric conversion element such as a CCD or CMOS. The center of gravity of the incident angle distribution of the light beam incident on the diffraction grating portion 10 (same as the center of gravity of the figure) is distributed from the center of the diffraction grating portion 10 with respect to the surface normal at the center of the diffraction grating of the envelope surface. Is set. In FIG. 5, the DOE 1 is provided on the front lens lens bonding surface. However, the present invention is not limited to this, and it may be provided on the lens surface, or a plurality of diffractive optical elements may be used in the photographing lens. .

  The optical system to which the DOE 1 can be applied is not limited to the photographing optical system shown in FIG. 5, but an imaging optical system and binoculars used in a wide wavelength region such as a photographing lens of a video camera, an image scanner, and a reader lens of a copying machine. Or an observation optical system such as a telescope or an optical viewfinder. An apparatus to which the optical system including the DOE 1 can be applied is not limited to the imaging apparatus, and may be a wide range of optical apparatuses.

  In FIGS. 2 and 6, off-screen light beams B and B ′ incident on the optical axis O at an incident angle ω pass through the substrate 2 and are respectively counted upward from the optical axis O in the mth and downward directions. The light enters the m-th and m′-th gratings that are the m-th diffraction grating. Incident angles of the off-screen light beams B and B ′ with respect to the m-grating and m′-grating are angles ωi and ωi ′ with respect to the principal ray direction. The lattice wall surfaces 1b and 1b 'are equal to the principal ray direction.

  FIG. 7 is a graph showing RCWA calculation results at an incident angle of +10 degrees, a grating pitch of 100 μm, and a wavelength of 550 nm, assuming the incident light beam b shown in FIG. 4 and the incident light beam B shown in FIG. The incident angle has a downward direction in FIG. 4 as a positive direction.

  FIG. 7A shows the diffraction efficiency in the vicinity of the + 1st order diffracted light, which is the designed order, where the horizontal axis represents the diffraction order and the vertical axis represents the diffraction efficiency. FIG. 7B is a result of enlarging the low diffraction efficiency portion of the vertical axis in FIG. 7A and displaying the high diffraction angle range with the horizontal axis as the diffraction order to the diffraction angle. The diffraction angle has a downward direction in FIG. 4 as a positive direction.

  As shown in FIG. 7A, the diffraction efficiency of the + 1st order diffracted light, which is the designed order, is concentrated, but the influence is small because the + 1st order diffracted light does not reach the image plane. The remaining unnecessary light propagates as unnecessary light having a peak in a specific angle direction as shown in FIG.

  This unnecessary light has a peak in a direction of approximately −10 degrees, and this propagation direction is substantially equal to the exit direction in which the component of the off-screen incident angle +10 degrees light beam incident on the grating wall surface is propagated with total reflection and −10 degrees.

  FIG. 8 is a graph as a comparative example corresponding to FIG. 7 when a DOE having the same configuration as that of FIG. 1 is used except that the thin film 20 is not provided.

  In this comparative example, as indicated by a light beam b1 in FIG. 9, a light beam b incident on the grating wall surface from the high refractive index material side to the low refractive index material side at a critical angle of 74.2 degrees or more at +80 degrees is The unnecessary light spreads from a direction of approximately −10 degrees to a high angle range (near the diffraction angle of 0 degrees). Since the diffraction angle 0 degree is substantially equal to the diffraction angle 0.20 degree of the + 1st order diffracted light at the designed incident angle 0 degree (+ 1st order light in FIG. 3), out of the unnecessary light with off-screen light +10 degrees incident, the diffraction angle +0. Unnecessary light emitted near 20 degrees reaches the image plane.

  Note that the diffraction order and the diffraction angle at which the unnecessary light of the off-screen incident light reaches the image plane differ depending on the optical system subsequent to the diffractive optical element. However, in any optical system, the diffracted light of unnecessary light due to off-screen light that substantially matches the diffraction angle at which the design diffraction order propagates at least at the design incident angle reaches the image plane, leading to a decrease in image performance. become.

  The unnecessary light peak angle in the −10 degree direction shown in FIG. 7B is almost the same as that in FIG. 8B, but the spread of the unnecessary light is different between FIG. 7B and FIG. ) Has lower diffraction efficiency at low diffraction angles.

  That is, in this embodiment, unnecessary light having a low diffraction angle (light beam b1 in FIG. 9) is reduced. In the optical system shown in FIGS. 5 and 6, diffracted light of unnecessary light due to off-screen light that substantially matches the diffraction angle +0.20 degree at which the design diffraction order at the design incident angle propagates at least reaches the image plane. From the RCWA calculation results, the diffraction efficiency in the vicinity of the diffraction angle +0.20 degrees in FIG. 7 is 0.0084% for the diffraction order −46th order (diffraction angle +0.34 degree), and the diffraction order −47th order (diffraction angle +0). .14 degrees) is 0.0083%.

  On the other hand, in the comparative example having no thin film, the diffraction efficiency of the diffraction order −46th order (diffraction angle + 0.34 degree) is 0.014%, and the diffraction efficiency of the diffraction order −47th order (diffraction angle + 0.14 degree) is. Since it is 0.014%, in this embodiment, the influence of unnecessary light is greatly reduced.

  FIG. 10 is a schematic diagram corresponding to FIG. 9 in the present embodiment. In this embodiment, unnecessary light is a result that a part of the light beam b incident near the grating wall surface is confined inside the thin film 20 and propagates as a light beam b3 like an optical waveguide, and these light beams interfere after exit. It is considered that the light flux reaching the image plane is reduced as compared with the comparative example.

  Next, the effect of incident light beams a and c shown in FIG. 4 will be described.

  FIG. 11 is a graph showing a result of RCWA calculation performed at an incident angle of 0 degree, a grating pitch of 100 μm, and a wavelength of 550 nm assuming the incident light beam a shown in FIG.

  FIG. 11A shows the diffraction efficiency in the vicinity of the + 1st order diffracted light, which is the designed order. The horizontal axis represents the diffraction order, and the vertical axis represents the diffraction efficiency. FIG. 11B is a result of enlarging the low diffraction efficiency portion of the vertical axis in FIG. 11A and displaying the high diffraction angle range with the horizontal axis as the diffraction order to the diffraction angle. The diffraction angle has a downward direction in FIG. 4 as a positive direction.

  FIG. 12 is a graph as a comparative example corresponding to FIG. 11 when a DOE having the same configuration as that of FIG. 1 is used except that the thin film 20 is not provided.

  The diffraction efficiency of the + 1st order diffracted light that is the designed order from FIG. 11A is 99.02% (diffraction angle +0.20 degrees), and +1 next time in the case of the diffraction grating without the thin film of FIG. The diffraction efficiency of folding light is equal to or higher than 98.76% (diffraction angle +0.20 degree). It can be seen that the remaining light becomes unnecessary light and propagates as shown in FIG. The lattice pitch assumed here is 100 μm as one reference. As shown in FIG. 1, the closer to the optical axis, the larger the grating pitch, and the adverse effect of the grating wall surface and reflecting means is reduced. Therefore, the diffraction efficiency of the designed order is high and the diffraction efficiency of unnecessary light is low.

  As a result, when the entire diffractive optical element is considered, the difference in diffraction efficiency of 0.26% with a grating pitch of 100 μm is that a high-intensity light source such as the sun is directly photographed at the design incident angle (photographing light incident angle). Is rare and has little effect and is not a problem. At the same time, the influence of unnecessary light is small.

  Next, FIG. 13 is a graph showing a result of RCWA calculation performed at an incident angle of −10 degrees, a grating pitch of 100 μm, and a wavelength of 550 nm assuming the incident light beam c shown in FIG. The incident angle has a downward direction in FIG. 4 as a positive direction (upward is a positive direction in the m ′ grating in FIG. 2).

  FIG. 13A shows the diffraction efficiency in the vicinity of the + 1st order diffracted light, which is the designed order, where the horizontal axis represents the diffraction order and the vertical axis represents the diffraction efficiency. FIG. 13B is a result of enlarging the low diffraction efficiency portion of the vertical axis in FIG. 13A and displaying the high diffraction angle range with the horizontal axis as the diffraction order to the diffraction angle.

  FIG. 14 is a graph as a comparative example corresponding to FIG. 13 when a DOE having the same configuration as that of FIG. 1 is used except that the thin film 20 is not provided.

  Although the diffraction efficiency of the + 1st order diffracted light, which is the designed order, is concentrated from FIG. 13A, this + 1st order diffracted light does not reach the image plane, so its influence is small. It can be seen that the remaining unnecessary light propagates as unnecessary light having a peak in a specific angle direction as shown in FIG. Compared to FIG. 14B, the peak of unnecessary light in the + direction increases and the peak of unnecessary light in the − direction decreases. This is because the high refractive index thin film provided on the grating wall surface reflects a part of the light beam incident on the grating wall surface from the low refractive index medium side, which increases unnecessary light in the + direction and is unnecessary due to transmission in the-direction. It means that light is decreasing.

  In the optical system shown in FIGS. 5 and 6, diffracted light of unnecessary light due to off-screen light that substantially matches the diffraction angle +0.20 degree at which the design diffraction order at the design incident angle propagates at least reaches the image plane. From the RCWA calculation results, the diffraction efficiency in the vicinity of the diffraction angle +0.20 degree in FIG. 13 is 0.010% for the diffraction order + 49th order (diffraction angle +0.26 degree), and the diffraction order + 48th order (diffraction angle +0.06). Degree) is 0.010%. In the case of the comparative example, the diffraction efficiency of the diffraction order + 49th order (diffraction angle +0.26 degrees) is 0.0021%, and the diffraction efficiency of the diffraction order + 48th order (diffraction angle +0.06 degrees) is 0.0022%. is there. As described above, the number of the present invention is increased compared to the comparative example, but since the numerical value of the diffraction efficiency is extremely small, the influence on the deterioration of the image performance is small.

  As described above, in the optical system to which the diffractive optical element of the present invention is applied, by providing a thin film on the grating wall surface, an increase in unnecessary light of the m ′ grating, which is less affected by unnecessary light, is suppressed to an extent that does not affect the unnecessary light. Therefore, it is possible to greatly reduce the unnecessary light of the m-grating which is greatly influenced by As a result, unnecessary light that reaches the imaging surface is reduced, so that deterioration in image performance can be suppressed. At the same time, the reduction in the diffraction efficiency of the designed order can be suppressed to the extent that the image performance is not affected.

  Here, the lattice pitch is 100 μm. Further, since the contribution of the wall surface is small in an annular zone with a wide grating pitch, the diffraction efficiency of the designed order is high and the diffraction efficiency of unnecessary light is low. Although not shown, the propagation direction of the unnecessary light does not depend on the grating pitch, and the propagation direction is the same. Therefore, the diffraction efficiency with a grating pitch of 100 μm is shown as one reference.

  Further, here, it is assumed that the incident angles of the off-screen light beams B and B ′ are +10 degrees outside the screen (the incident angle ω is +13.16 degrees with respect to the optical axis direction). At angles smaller than this incident angle, ghosts due to lens surface and image plane reflection, scattering inside the lens, and surface irregularities are large, and therefore unnecessary light of the diffractive optical element is relatively inconspicuous. At an angle larger than this incident angle, the influence of unnecessary light of the diffractive optical element is relatively small due to reflection of the front lens surface and light shielding by the lens barrel. For this reason, the off-screen incident light flux has the greatest influence on the unnecessary light of the diffractive optical element in the vicinity of +10 degrees. Here, the incident angle of the off-screen light flux is assumed to be approximately +10 degrees.

  In this embodiment, two diffraction gratings are arranged in close contact, and the materials constituting the diffraction gratings and the heights of the diffraction gratings are set appropriately to achieve high diffraction efficiency in a wide wavelength band with respect to a predetermined order of diffracted light. Realized.

  In the present embodiment, unnecessary light reaching the imaging plane can be reduced by satisfying the following expression in the DOE 1. Here, nd2 is the refractive index with respect to the d-line of the material constituting the diffraction grating 12, and nd3 is the (maximum) refractive index with respect to the d-line of the material constituting one layer of the thin film 20.

  In the present embodiment, an example is described in which nd2 is larger than the refractive index nd1 of the material constituting the diffraction grating 11 with respect to the d-line. However, when nd1> nd2, since the direction of the grating shape of the diffraction grating is reversed, the influence of unnecessary light by the grating wall surface is the same, and can be generalized as follows.

  In this example, the high refractive index thin film on the grating wall reduces the propagation of the electromagnetic field from the low refractive index material side to the high refractive index material side of the grating wall, and a part of the light beam is confined inside the high refractive index thin film. It is utilized that it is propagated like an optical waveguide. If Expression 1 is not satisfied, the difference in refractive index is reduced and the effect of reducing unnecessary light is also reduced. In this respect, since Patent Document 3 suppresses the difference in refractive index between one of the thin film and the two optical layers to 0.01 or less, the effect of this example cannot be obtained.

  In this embodiment, since the refractive index nd3 = 2.323, the refractive index nd2 = 1.567, and the refractive index nd1 = 1.504, nd3-nd2 = 0.756 and nd3-nd1 = 0.919.

  In this embodiment, as shown in FIG. 6, the peak of unnecessary light is shielded by the diaphragm 40, but the present invention is not limited to this.

  Example 2 is the same as Example 1, except that the width w of the thin film is not 0.1 μm but 1.0 μm.

  FIG. 15 is a graph showing RCWA calculation results for the incident light beam b shown in FIG. 4 and the incident light beam B shown in FIG. 6 at an incident angle of +10 degrees, a grating pitch of 100 μm, and a wavelength of 550 nm.

  The diffraction efficiency of the + 1st order diffracted light, which is the designed order, is concentrated, but the influence is small because the + 1st order diffracted light does not reach the image plane. The remaining unnecessary light propagates as unnecessary light having a peak in a specific angle direction as in the first embodiment.

  Although the unnecessary light peak angle in the -10 degree direction is almost the same as that in FIG. 8B, the spread of unnecessary light is different in FIGS. 15 and 8B, and FIG. 15 has lower diffraction efficiency at a low diffraction angle. That is, in this embodiment, unnecessary light having a low diffraction angle (light beam b1 in FIG. 9) is reduced.

  In the optical system shown in FIGS. 5 and 6, diffracted light of unnecessary light due to off-screen light that substantially matches the diffraction angle +0.20 degree at which the design diffraction order at the design incident angle propagates at least reaches the image plane. From the RCWA calculation result, the diffraction efficiency in the vicinity of the diffraction angle +0.20 degree in FIG. 15 is 0.0065% for the diffraction order −46th order and 0.0074% for the diffraction order −47th order. Similar to FIG. 7B, the diffraction efficiency is greatly reduced.

  FIG. 16 is a graph showing a result of RCWA calculation performed at an incident angle of 0 degree, a grating pitch of 100 μm, and a wavelength of 550 nm assuming the incident light beam a shown in FIG.

  The diffraction efficiency of the + 1st order diffracted light that is the designed order is 96.06%, which is lower than the diffraction efficiency of the + 1st order diffracted light in the case of a diffraction grating not provided with a thin film. It can be seen that the remaining light becomes unnecessary light and propagates in the same manner as in the first embodiment. As a result of the phase mismatch caused by this thin film, the diffraction efficiency of the relatively low order (approximately ± 35th order, diffraction angle ± 10 degrees) increases, and the diffraction efficiency of the + 1st order diffracted light, which is the designed order, decreases. ing. Further, since the width of the thin film is thicker than that of the first embodiment, the amount of reduction in the diffraction efficiency of the + 1st order diffracted light is larger than that of the first embodiment.

  Considering the entire area of the diffractive optical element, the reduction amount of the diffraction efficiency of 2.70% with a grating pitch of 100 μm is rarely taken directly by a high-intensity light source such as the sun during the day at the designed incident angle (photographing light incident angle). Therefore, it has almost no effect and is not a problem. At the same time, the influence of unnecessary light is small.

  Next, FIG. 17 is a graph showing a result of RCWA calculation performed at an incident angle of −10 degrees, a grating pitch of 100 μm, and a wavelength of 550 nm assuming the incident light beam c shown in FIG.

  Although the diffraction efficiency of the + 1st order diffracted light, which is the designed order, is concentrated from FIG. 17, the influence is small because the + 1st order diffracted light does not reach the image plane. It can be seen that the remaining unnecessary light is propagated as unnecessary light having a peak in a specific angle direction as in the first embodiment. As shown in FIGS. 2, 5, and 6, diffracted light of unnecessary light due to off-screen light that substantially matches the diffraction angle +0.20 degree at which the design diffraction order at the design incident angle propagates at least reaches the image plane. The diffraction efficiency in the vicinity of the diffraction angle +0.20 degree in FIG. 17 is 0.0060% for the diffraction order + 49th order and 0.0064% for the diffraction order + 48th order from the RCWA calculation result. Although it is increased as compared with the case of the diffraction grating not provided with a thin film, since the numerical value of the diffraction efficiency is extremely small, the influence on the deterioration of the image performance is small.

  As described above, in the optical system to which the diffractive optical element of the present invention is applied, by providing a thin film, an increase in unnecessary light of the m ′ grating, which is less affected by unnecessary light, is suppressed to an extent that does not affect the influence of unnecessary light. Unnecessary light of a large m grating can be greatly reduced. As a result, unnecessary light that reaches the imaging surface is reduced, so that deterioration in image performance can be suppressed. At the same time, the reduction in the diffraction efficiency of the designed order can be suppressed to the extent that the image performance is not affected.

  Also in this embodiment, nd3 = 2.323, nd2 = 1.567, and nd1 = 1.504, so nd3-nd2 = 0.756 and nd3-nd1 = 0.919, satisfying Equation 1. Yes.

  As described above, also in this embodiment, in the optical system to which DOE 1 is applied, the unnecessary light that the thin film 20 reaches the imaging surface is reduced to suppress the deterioration of the image performance, and the diffraction efficiency of the design order does not affect the image performance. Can be suppressed.

  Although it is not limited to the thickness of the thin film 20 as shown in the present embodiment, when the width is increased, the phase mismatch region of the diffraction grating 11 and the diffraction grating 12 is expanded, and unnecessary diffraction light of a relatively low order is expanded. The diffraction efficiency increases and the diffraction efficiency (image performance) of the designed order decreases.

  Therefore, it is only necessary to satisfy the following equation that the value obtained by dividing the total thickness (width) W of the thin film by the grating pitch of the diffractive optical element is greater than 0 and less than 0.05. Here, P is the lattice pitch, and W is the total thickness in the direction perpendicular to the lamination surface of the thin film 20 (the total thickness of each layer if the thin film is a multilayer structure).

  Regarding the diffraction efficiency of the design order, the relationship between the width w of the thin film and the grating pitch P is linear, and the diffraction efficiency of the design order of the diffraction grating having the width w of the thin film 20 and the grating pitch P and the width w × 2 of the thin film 20 The diffraction efficiency of the design order of the diffraction grating having the grating pitch P × 2 is substantially the same.

  For example, the diffraction efficiency of the designed orders of the diffraction grating having the grating pitch of 100 μm and the thin film width of 1.0 μm and the diffraction pitch of 200 μm and the thin film width of 2.0 μm shown in Example 1 are substantially the same. . For this reason, Formula 2 is materialized.

  Further, when the width of the thin film 20 is increased, leakage light due to an increase in the number of propagation modes in the optical waveguide is increased. When the maximum thickness w of the thin film having the maximum refractive index is 2 μm or more, unnecessary light is generated. Since the ripple increases, the flare reduction effect decreases. Therefore, it is preferably less than 2 μm. If the thin film has a multilayer structure, it means that the thickness of the layer having the maximum thickness is less than 2 μm.

The third embodiment is different from the first embodiment in that the thin film 20 is Ta ₂ O ₅ (n = 2.154), and is otherwise similar to the first embodiment.

  FIG. 18 is a graph showing RCWA calculation results for the incident light beam b shown in FIG. 4 and the incident light beam B shown in FIG. 6 at an incident angle of +10 degrees, a grating pitch of 100 μm, and a wavelength of 550 nm.

  The diffraction efficiency of the + 1st order diffracted light, which is the designed order, is concentrated, but the influence is small because the + 1st order diffracted light does not reach the image plane. The remaining unnecessary light propagates as unnecessary light having a peak in a specific angle direction as in the first embodiment.

  Although the unnecessary light peak angle in the -10 degree direction is almost the same as that in FIG. 8B, the spread of unnecessary light is different in FIGS. 18 and 8B, and FIG. 18 has lower diffraction efficiency at a low diffraction angle. That is, in this embodiment, unnecessary light having a low diffraction angle (light beam b1 in FIG. 9) is reduced.

  In the optical system shown in FIGS. 5 and 6, diffracted light of unnecessary light due to off-screen light that substantially matches the diffraction angle +0.20 degree at which the design diffraction order at the design incident angle propagates at least reaches the image plane. From the RCWA calculation results, the diffraction efficiency near the diffraction angle +0.20 degree in FIG. 18 is 0.0067% for the diffraction order −46th order and 0.0067% for the diffraction order −47th order. Similar to FIG. 7B, the diffraction efficiency is greatly reduced.

  FIG. 19 is a graph showing a result of RCWA calculation performed at an incident angle of 0 degree, a grating pitch of 100 μm, and a wavelength of 550 nm assuming the incident light beam a shown in FIG.

  The diffraction efficiency of the + 1st order diffracted light that is the designed order is 98.39%, which is lower than the diffraction efficiency of the + 1st order diffracted light in the case of a diffraction grating not provided with a thin film. It can be seen that the remaining light becomes unnecessary light and propagates in the same manner as in the first embodiment. As a result of the phase mismatch caused by this thin film, the diffraction efficiency of the relatively low order (approximately ± 35th order, diffraction angle ± 10 degrees) increases, and the diffraction efficiency of the + 1st order diffracted light, which is the designed order, decreases. ing.

  Considering the entire area of the diffractive optical element, the reduction amount of 0.37% of diffraction efficiency with a grating pitch of 100 μm is rarely taken directly by a high-intensity light source such as the sun at the design incident angle (photographing light incident angle). Therefore, it has almost no effect and is not a problem. At the same time, the influence of unnecessary light is small.

  Next, FIG. 20 is a graph showing a result of RCWA calculation performed at an incident angle of −10 degrees, a grating pitch of 100 μm, and a wavelength of 550 nm assuming the incident light beam c shown in FIG.

  Although the diffraction efficiency of the + 1st order diffracted light, which is the designed order, is concentrated from FIG. 20, this + 1st order diffracted light does not reach the image plane, so its influence is small. It can be seen that the remaining unnecessary light is propagated as unnecessary light having a peak in a specific angle direction as in the first embodiment. As shown in FIGS. 2, 5, and 6, diffracted light of unnecessary light due to off-screen light that substantially matches the diffraction angle +0.20 degree at which the design diffraction order at the design incident angle propagates at least reaches the image plane. From the RCWA calculation results, the diffraction efficiency in the vicinity of the diffraction angle +0.20 degree in FIG. 20 is 0.0084% for the diffraction order + 49th order and 0.0084% for the diffraction order + 48th order. Although it is increased as compared with the case of the diffraction grating not provided with a thin film, since the numerical value of the diffraction efficiency is extremely small, the influence on the deterioration of the image performance is small.

  As described above, in the optical system to which the diffractive optical element of the present invention is applied, by providing a thin film, an increase in unnecessary light of the m ′ grating, which is less affected by unnecessary light, is suppressed to an extent that does not affect the influence of unnecessary light. Unnecessary light from a large m-grating can be greatly reduced. As a result, unnecessary light that reaches the imaging surface is reduced, so that deterioration in image performance can be suppressed. At the same time, the reduction in the diffraction efficiency of the designed order can be suppressed to the extent that the image performance is not affected.

  Also in this example, nd3 = 2.154, nd2 = 1.567, and nd1 = 1.504, so nd3-nd2 = 0.487, nd3-nd1 = 0.650, and satisfying Equation 1. Yes.

  As described above, in the optical system of the present embodiment, it is possible to reduce the unnecessary light that the thin film 20 reaches the image formation surface to suppress the deterioration of the image performance, and to suppress the diffraction efficiency of the design order to an extent that does not affect the image performance. it can.

  Example 4 is an example in which the thin film is provided not only on the grating wall surface but on the entire boundary surface (the thin film 22 is provided continuously from the grating wall surface to the grating surface), and FIG. 21 is an enlargement of the diffraction grating of Example 4. It is sectional drawing. FIG. 21 is a diagram that is considerably deformed in the lattice period direction so that it can be easily understood. The configurations of the diffraction grating 11 and the diffraction grating 12 are the same as those in the first embodiment.

  A transparent thin film 21 is provided over the entire boundary surface between the diffraction grating 11 and the diffraction grating 12, and the thin film 21 has a substantially uniform thickness from the grating surface to the entire area of the grating wall surface. The thin film 21 controls the unnecessary light generated by the antireflection function and the obliquely incident (off-screen incident) light beam with respect to the vertically incident light beam (incident on the screen) incident on the grating surface, and the unnecessary light reaching the imaging surface. It has a function to reduce

The thin film 21 is formed of a multilayer film having 15H, 27L, 120H, 28L, and 15H in order from the diffraction grating 11 to the diffraction grating 12. Here, H represents a high refractive index layer (TiO ₂ layer), L represents a low refractive index layer (SiO ₂ (n = 1.482) layer), and a numerical value represents a physical film thickness (nm). Of the five thin films, one high refractive index thin film is designed to be physically thicker than the other thin films. The transmittance of the entire visible region (430 nm to 670 nm) is designed to be 99.7% or more with respect to an incident angle of 0 degree (incident light beam a) which is a designed incident angle on the grating surface of the diffractive optical element. Yes.

  FIG. 22 is a graph showing RCWA calculation results for the incident light beam b shown in FIG. 21 and the incident light beam B shown in FIG. 6 at an incident angle of +10 degrees, a grating pitch of 100 μm, and a wavelength of 550 nm.

  The diffraction efficiency of the + 1st order diffracted light, which is the designed order, is concentrated, but the influence is small because the + 1st order diffracted light does not reach the image plane. Similar to the first embodiment, the remaining unnecessary light propagates as unnecessary light having a peak in a specific angle direction.

  Although the unnecessary light peak angle in the −10 degree direction is almost the same as that in FIG. 8B, the spread of unnecessary light is different in FIGS. 22 and 8B, and FIG. 22 has a lower diffraction efficiency at a low diffraction angle. That is, in this embodiment, unnecessary light having a low diffraction angle (light beam b1 in FIG. 9) is reduced.

  In the optical system shown in FIGS. 5 and 6, diffracted light of unnecessary light due to off-screen light that substantially matches the diffraction angle +0.20 degree at which the design diffraction order at the design incident angle propagates at least reaches the image plane. The diffraction efficiency in the vicinity of the diffraction angle +0.20 degree in FIG. 22 is 0.010% for the diffraction order −46th order and 0.010% for the diffraction order −47th order from the RCWA calculation result. Similar to FIG. 7B, the diffraction efficiency is greatly reduced.

  FIG. 23 is a graph showing a result of RCWA calculation performed at an incident angle of 0 degree, a grating pitch of 100 μm, and a wavelength of 550 nm assuming the incident light beam a shown in FIG.

  The diffraction efficiency of the + 1st order diffracted light that is the designed order is 98.61%, which is lower than the diffraction efficiency of the + 1st order diffracted light in the case of a diffraction grating not provided with a thin film. It can be seen that the remaining light becomes unnecessary light and propagates in the same manner as in the first embodiment. As a result of the phase mismatch caused by this thin film, the diffraction efficiency of the relatively low order (approximately ± 35th order, diffraction angle ± 10 degrees) increases, and the diffraction efficiency of the + 1st order diffracted light, which is the designed order, decreases. ing. Considering the entire area of the diffractive optical element, the amount of reduction of the diffraction efficiency of 0.15% with a grating pitch of 100 μm is rarely taken directly by a high-intensity light source such as the sun during the day at the designed incident angle (photographing light incident angle). Therefore, it has almost no effect and is not a problem. At the same time, the influence of unnecessary light is small.

  Next, FIG. 24 is a graph showing a result of RCWA calculation performed at an incident angle of −10 degrees, a grating pitch of 100 μm, and a wavelength of 550 nm, assuming the incident light beam c shown in FIG.

  Although the diffraction efficiency of the + 1st order diffracted light, which is the designed order, is concentrated from FIG. 24, the influence is small because the + 1st order diffracted light does not reach the image plane. It can be seen that the remaining unnecessary light is propagated as unnecessary light having a peak in a specific angle direction as in the first embodiment. As shown in FIGS. 2, 5, and 6, diffracted light of unnecessary light due to off-screen light that substantially matches the diffraction angle +0.20 degree at which the design diffraction order at the design incident angle propagates at least reaches the image plane. The diffraction efficiency in the vicinity of the diffraction angle +0.20 degree in FIG. 24 is 0.011% for the diffraction order + 49th order and 0.011% for the diffraction order + 48th order from the RCWA calculation result. Although it is increased as compared with the case of the diffraction grating not provided with a thin film, since the numerical value of the diffraction efficiency is extremely small, the influence on the deterioration of the image performance is small.

  As described above, in the optical system to which the diffractive optical element of the present invention is applied, by providing a thin film, an increase in unnecessary light of the m ′ grating, which is less affected by unnecessary light, is suppressed to an extent that does not affect the influence of unnecessary light. Unnecessary light from a large m-grating can be greatly reduced. As a result, unnecessary light that reaches the imaging surface is reduced, so that deterioration in image performance can be suppressed. At the same time, the reduction in the diffraction efficiency of the designed order can be suppressed to the extent that the image performance is not affected.

  Moreover, although the film | membrane structure of the thin film 21 of a present Example is five layers, it is not limited about the number of layers, a film thickness, and film | membrane material, A single layer thin film as shown in Examples 1-3 may be sufficient. The film configuration can arbitrarily design the antireflection characteristics of the grating surface and the unnecessary light suppression effect of the grating wall surface by selecting the materials of the diffraction gratings 11 and 12. In the multilayer thin film, it is preferable that the thin film of the high refractive index material is optically thickest to form an optical waveguide structure.

  Conversely, in Examples 1 to 3, a single layer film is provided on the lattice wall surface, but a multilayer thin film may be provided on the lattice wall surface. Also in this case, it is preferable that the thin film of the high refractive index material is optically thickest in the multilayer thin film so as to have an optical waveguide structure.

  Since this embodiment forms a thin film over the entire boundary surface, a diffractive optical element can be manufactured more easily and at a lower cost than in the first to third embodiments. In the manufacture of the diffractive optical element, for example, after the diffraction grating 12 is manufactured, a thin film is formed from the grating surface to the entire grating wall surface by vacuum deposition or the like, and then the diffraction grating 11 is formed. Not. Furthermore, the adhesion between the diffraction grating 11 and the diffraction grating 12 can be improved by providing a thin film over the entire interface.

  Example 5 is the same as Example 4, but the total thickness on the lattice plane of the thin film is different from the total thickness on the grid wall surface (the total thickness of the thin film depends on the position of the boundary surface). Is different from the fourth embodiment. FIG. 25 is an enlarged cross-sectional view of the diffraction grating of Example 4. FIG. 25 is a diagram that is considerably deformed in the lattice period direction so that it can be easily understood.

The thin film 21 is composed of a multilayer film having 20H, 19L, 229H, 22L, and 19H in this order from the diffraction grating 11 to the diffraction grating 12. Here, H represents a high refractive index layer (TiO ₂ layer), L represents a low refractive index layer (SiO ₂ layer), and a numerical value represents a physical film thickness (nm). Of the five thin films, one high refractive index thin film is designed to be thicker than the other thin films. The film thickness of the thin film 21 on the grating wall surface is set to half of the physical film thickness. Specifically, the thin film 21 has 10H, 10L, 115H, 11L, and 10H in order from the diffraction grating 11 to the diffraction grating 12. . Of the five thin films, one high refractive index thin film is designed to be physically thicker than the other thin films. The transmittance of the entire visible region (430 nm to 670 nm) is designed to be 99.0% or more with respect to an incident angle of 0 degree (incident light beam a) which is a designed incident angle on the grating surface of the diffractive optical element. Yes.

  FIG. 26 is a graph showing RCWA calculation results for the incident light beam b shown in FIG. 25 and the incident light beam B shown in FIG. 6 at an incident angle of +10 degrees, a grating pitch of 100 μm, and a wavelength of 550 nm.

  The diffraction efficiency of the + 1st order diffracted light, which is the designed order, is concentrated, but the influence is small because the + 1st order diffracted light does not reach the image plane. The remaining unnecessary light propagates as unnecessary light having a peak in a specific angle direction as in the first embodiment.

  The unnecessary light peak angle in the −10 degree direction is almost the same as that in FIG. 8B, but the spread of unnecessary light is different in FIGS. 26 and 8B, and FIG. 26 has lower diffraction efficiency at a low diffraction angle. That is, in this embodiment, unnecessary light having a low diffraction angle (light beam b1 in FIG. 9) is reduced.

  In the optical system shown in FIGS. 5 and 6, diffracted light of unnecessary light due to off-screen light that substantially matches the diffraction angle +0.20 degree at which the design diffraction order at the design incident angle propagates at least reaches the image plane. The diffraction efficiency in the vicinity of the diffraction angle +0.20 degree in FIG. 26 is 0.010% for the diffraction order −46th order and 0.010% for the diffraction order −47th order from the RCWA calculation result. Similar to FIG. 7B, the diffraction efficiency is greatly reduced.

  FIG. 27 is a graph showing a result of RCWA calculation performed at an incident angle of 0 degree, a grating pitch of 100 μm, and a wavelength of 550 nm assuming the incident light beam a shown in FIG.

  The diffraction efficiency of the + 1st order diffracted light that is the designed order is 97.90%, which is lower than the diffraction efficiency of the + 1st order diffracted light in the case of a diffraction grating not provided with a thin film. It can be seen that the remaining light becomes unnecessary light and propagates in the same manner as in the first embodiment. As a result of the phase mismatch caused by this thin film, the diffraction efficiency of the relatively low order (approximately ± 35th order, diffraction angle ± 10 degrees) increases, and the diffraction efficiency of the + 1st order diffracted light, which is the designed order, decreases. ing.

  Considering the entire area of the diffractive optical element, the reduction amount of the diffraction efficiency of 0.86% when the grating pitch is 100 μm is rarely taken directly by a high-intensity light source such as the sun during the day at the designed incident angle (photographing light incident angle). Therefore, it has almost no effect and is not a problem. At the same time, the influence of unnecessary light is small.

  Next, FIG. 28 is a graph showing a result of RCWA calculation performed at an incident angle of −10 degrees, a grating pitch of 100 μm, and a wavelength of 550 nm assuming the incident light beam c shown in FIG.

  Although the diffraction efficiency of the + 1st order diffracted light, which is the designed order, is concentrated from FIG. 28, the influence is small because the + 1st order diffracted light never reaches the image plane. It can be seen that the remaining unnecessary light is propagated as unnecessary light having a peak in a specific angle direction as in the first embodiment. As shown in FIGS. 2, 5, and 6, diffracted light of unnecessary light due to off-screen light that substantially matches the diffraction angle +0.20 degree at which the design diffraction order at the design incident angle propagates at least reaches the image plane. The diffraction efficiency in the vicinity of the diffraction angle +0.20 degree in FIG. 28 is 0.013% for the diffraction order + 49th order and 0.013% for the diffraction order + 48th order from the RCWA calculation result. Although it is increased as compared with the case of the diffraction grating not provided with a thin film, since the numerical value of the diffraction efficiency is extremely small, the influence on the deterioration of the image performance is small.

  As described above, in the optical system to which the diffractive optical element of the present invention is applied, by providing a thin film, an increase in unnecessary light of the m ′ grating, which is less affected by unnecessary light, is suppressed to an extent that does not affect the influence of unnecessary light. Unnecessary light from a large m-grating can be greatly reduced. As a result, unnecessary light that reaches the imaging surface is reduced, so that deterioration in image performance can be suppressed. At the same time, the reduction in the diffraction efficiency of the designed order can be suppressed to the extent that the image performance is not affected.

  Since the film thickness of the grating surface and the grating wall surface may be different as in this example, this example can manufacture a diffractive optical element more easily and inexpensively. For example, when forming a thin film by vacuum deposition, the thickness of the grating surface of the blazed grating, which is a saw shape, is generally different from the film thickness of the grating wall surface, and a diffraction grating is formed on the lens surface as shown in FIG. The film thickness also differs when For this reason, it is possible to achieve both the reduction of the diffraction efficiency of the design order and the reduction of unnecessary light due to the off-screen light beam by arbitrarily designing the antireflection function of the grating surface and the flare reduction function of the grating wall surface according to the manufacturing method. . An example of a manufacturing method is the same as that of Example 4.

  Table 1 summarizes the results of Examples 1-5. nd1 is the refractive index of the d-line of the diffraction grating 11, nd2 is the refractive index of the d-line of the diffraction grating 12, vd1 is the Abbe number of the diffraction grating 11, vd2 is the Abbe number of the diffraction grating 12, and θgF1 is the partial dispersion of the diffraction grating 11. The ratio θgF2 is the partial dispersion ratio of the diffraction grating 12. n1_550 is the refractive index of the diffraction grating 11 at a wavelength of 550 nm, n2_550 is the refractive index of the diffraction grating 12 at a wavelength of 550 nm, d is the grating height of the diffractive optical element, W is the material constituting the thin film, the film thickness, and the entire width of the thin film It is. This is the width w of the maximum film thickness among the thin films made of one material. The diffraction efficiency (%) in the table is a calculation result of RCWA of diffraction orders of −46th order and −47th order at an incident angle of +10 degrees corresponding to the incident light beam B and a grating pitch of 100 μm. In addition, about Example 5, the film thickness on a lattice plane is shown.

  As shown in Examples 1 to 5, the refractive indexes nd1 and nd2 of the diffraction gratings 11 and 12 that are in close contact with each other through the thin film and the refractive index nd3 with respect to the d-line of at least one material of the thin film satisfy Equation 1. Therefore, unnecessary light reaching the imaging plane can be reduced. In addition, the thickness of the thin film only needs to satisfy Formula 2 in order to prevent a decrease in image performance. Further, in order to prevent the flare reduction effect from decreasing, it is sufficient to satisfy Equation 3.

  In Examples 1 to 5, the refractive index relationship between the diffraction grating 11 and the diffraction grating 12 satisfies the following expression. Since the transmittance is 99% or more with this refractive index difference, it is not usually necessary to provide an antireflection film at the interface. However, in Examples 1 to 5, unnecessary light is reduced by providing an antireflection film. .

  In this embodiment, nd2−nd1 = 0.063, which corresponds to the case where the reflectance between the diffraction gratings 11 and 12 is 1% or less.

  In the present embodiment, an example is described in which nd2 is larger than the refractive index nd1 of the material constituting the diffraction grating 11 with respect to the d-line. However, in general, Equation 4 is obtained by subtracting the smaller one from the larger one of the refractive index for the d-line of the material of the first diffraction grating and the refractive index for the d-line of the material of the second diffraction grating. It means larger than 0 and smaller than 0.223.

  Further, it is not always necessary to provide a thin film on all the annular zones, and they may be provided on a part of the annular zones. At this time, it is effective to provide a thin film in a part including the minimum lattice pitch. This is because a diffraction grating having a small grating pitch has a large diffraction efficiency of unnecessary light, and therefore the contribution of unnecessary light generated in the entire diffractive optical element is large.

  As mentioned above, although the preferable Example of this invention was described, this invention is not limited to these Examples, A various deformation | transformation and change are possible within the range of the summary.

  The diffractive optical element can be applied to an application requiring a diffractive action.

1 Diffraction optical element 11, 12 Diffraction grating 20, 21 Thin film

Claims (8)

A diffractive optical element used for a lens of an optical system,
A first diffraction grating and a second diffraction grating formed of different materials and stacked in the optical axis direction;
The first diffraction grating and the second diffraction grating are arranged in at least a part of a boundary surface between the first diffraction grating and the second diffraction grating, and the first diffraction grating and the second diffraction grating are composed of a single layer or multiple layers made of different materials. A thin film that is transparent to light in the wavelength band used,
Have
The refractive index nd1 of the first diffraction grating material with respect to the d-line, the refractive index nd2 of the material of the second diffraction grating with respect to the d-line, and the maximum refractive index nd3 with respect to the d-line of the material constituting one layer of the thin film. Satisfies the following formula: a diffractive optical element.
nd1 <nd2
0.5 <nd3-nd2 <0.8 2. The diffractive optical element according to claim 1, wherein the maximum thickness w of the layer having the maximum refractive index of the thin film and the total thickness W of the thin film satisfy the following expression.
0 <w <2 μm
0 <W / P <0.05 The diffractive optical element according to claim 1, wherein a refractive index of a material of the diffraction grating satisfies the following formula.
0 <nd2-nd1 <0.223   The diffractive optical element according to claim 1, wherein the thin film is provided only on a grating wall surface.   The said thin film has a multilayer which has a different refractive index, The thickness of a layer with a high refractive index is physically thicker than the thickness of a layer with a low refractive index, The any one of Claims 1-4 characterized by the above-mentioned. The diffractive optical element according to one item.   The diffractive optical element according to claim 1, wherein the thin film has a thickness that varies depending on a position of the boundary surface. The diffractive optical element according to any one of claims 1 to 6,
A stop disposed behind the diffractive optical element;
An optical system comprising:   An optical apparatus comprising the optical system according to claim 7.